Method to fabricate thin insulating film

ABSTRACT

In this disclosure, we present processes of growing SiO 2  films over silicon at temperatures as low as room temperature and at pressures as high as 1 atmosphere. The lower temperature oxidation was made possible by creation of oxygen atoms and radicals by adding noble gas(es) along with oxidizing gas(es) and applying RF power to create plasma. It was also possible to fabricate silicon nitride films by flowing nitrogen containing gas(es) with noble gas(es) and applying RF power to create plasma at pressures as high as one atmosphere. In addition, the above processes could also be carried out using microwave power instead of RF power to create plasma.

TECHNICAL FIELD

[0001] The present invention relates to a method of fabricatinginsulating films for application in to thin film transistors (TFTs) andmetal insulator oxide (MOS) transistors.

BACKGROUND ART

[0002] A process for forming an insulating film such as SiO₂ is one ofkey processes for manufacturing transistors such as silicon MOSFET(Metal Oxide Semiconductor Field Effect Transistor). Formation of SiO₂film on silicon is carried out at temperatures usually higher than 1000degree C. in the presence of chemical species that oxidize the silicon.This process is known as thermal oxidation. The thermal oxidationprocess has undesirable side-effects such as redistribution of thedopant profiles in the semiconductors since significant diffusion ofdopants occurs at high temperatures used in the this process.

[0003] Thin film transistor (TFT) devices, which have a basic structuresimilar to that of a usual MOSFET, have been used for displayapplications such as liquid crystal display (LCD) and organicelectroluminescence display (OELD). Such devices require a SiO₂ layer tobe formed at a temperature below 430 degree C., as these displays useoptically transparent substrate such as glass which can not withstandhigher temperatures. For such TFT applications, currently deposited SiO₂films are used which are of inferior quality and also form an inferiorinterface with silicon compared to SiO₂ produced by oxidation ofsilicon; thereby affecting the TFT performance adversely. Thus it isrequired that the oxidation process is carried out at temperatures aslow as possible.

[0004] Recently, Ueno et al (reference 1) and Ohmi et al (reference 2)reported that the application of microwave field to a gas mixture ofnoble gas and oxygen generates plasma containing atomic oxygen andoxygen radicals, which easily oxidize silicon to form an SiO₂ film evenat temperatures lower than 500 degree C.

[0005] The low temperature processes mentioned in the references aboveinvolve generation of plasma. Generally plasma process are carried outat pressures of 1 torr (133 Pa) or less. Additionally theprocess-chamber needs to be evacuated to a even lower pressure (basepressure) before the process gases are introduced in to the chamber.Thus these plasma processes require the use of expensive vacuum tools.The maintenance of the vacuum tools further adds to the cost. Vacuumtools also use up expensive clean room space. Additionally, themicrowave plasma processes used above are also limited in theirapplications. The microwave processes are suitable for semiconductorprocessing where the maximum size for the substrate is 300 mm indiameter. For the case of TFTs, the substrate size is much larger andapproaches 1000 mm×1000 mm in the latest generation equipment. For suchlarge substrate, the microwave plasma process is not suitable.

DISCLOSURE OF INVENTION

[0006] The object of the present invention is to provide an inexpensivemethod for fabricating insulating films of high quality at lowtemperatures. Further, it is the object of this invention that suchinsulating films can be fabricated over large substrate such as thoseused in the TFT fabrication.

[0007] In this disclosure, the inventor reports on the fabrication ofhigh quality SiO₂ films by plasma excitation of noble gases along withoxygen at the pressures substantially close to atmospheric-pressure(about 100 kPa). This completely eliminates the need of using vacuumtools, making the equipment and the process very inexpensive compared toequipment and processes used for making similar insulating films in TFTand semiconductor industries. Additionally, the inventor used radiofrequency (RF) power in the MHz range for this process, which makes itpossible to apply the process to large substrate.

[0008] Additionally, the same process could be used to form siliconnitride by using the nitriding species (such as NH₃, N₂ etc.) along withnoble gases and creating plasma at the pressures substantially close toatmospheric pressure.

[0009] The method of creating RF plasma at pressures substantially closeto atmospheric-pressure is advantageous from the cost and simplicitypoints of view. As a matter of course, even if the process pressure isreduced to as low as 1 kPa, the processes can be carried out withinexpensive vacuum tools.

BRIEF DESCRIPTION OF DRAWINGS

[0010]FIG. 1. A simple schematic of plasma oxidation process using RFpower

[0011]FIG. 2. C-V curve for the MOS capacitor fabricated using the SiO₂film grown using the disclosed method of example 1

[0012]FIG. 3. C-V curve for the MOS capacitor fabricated using the SiO₂film grown using the disclosed method of example 2

BEST MODE FOR CARRYING OUT THE INVENTION

[0013] A noble gas and a reagent gas (such as oxidizing agent) wereintroduced in to a chamber. The chamber contains two electrodes (seeFIG. 1) between which RF power was applied. The pressure in the chamberwas substantially close to one atmosphere (about 100 kPa) and no vacuumequipment was employed in this process. In the routine plasma processingcurrently being employed in the industry, which is done at pressures ofthe order of 100 Pa or lower, the formation of plasma is relativelyeasy. In these routine plasma processes, typically the RF power densityis of the order of several hundred milli-watts/cm², and the spacingbetween the electrodes is of the order of 20 mm. However, it becomesincreasingly difficult to form plasma as the pressure is increased. Andit has been almost impossible to create plasma at pressuressubstantially close to one atmospheric using only the reagent gases suchas oxygen. Addition of a noble gas such as helium or argon in largeproportion to the reagent gas makes it easier to form plasma at higherpressures, however to form plasma at pressure as high as atmosphericpressure is still quite difficult. In order to create plasma at thepressures substantially close to one atmospheric, RF power density andthe electrode spacing had to be changed significantly, in addition toadding a noble gas to a reagent gas. The inventor found that the RFpower density of several watts/cm² in the MHz range was necessary. Aplasma was more effectively formed when the distance between theelectrodes on which RF power was applied was less than 5 mm.

[0014] In addition to oxidation of silicon. the above process can alsobe applied to nitridation of silicon at pressures substantially close toone atmosphere. The nitridation can be carried out by in the presence ofnitrogen containing-compounds (such as N₂, NH₃ etc.) instead of oxygencontaining compounds along with noble gas(s) and sustaining plasma.These nitride films can be subjected to further processing such asannealing (thermal annealing, rapid thermal annealing, laser annealing)to further improve the quality of the films.

[0015] It should also be noted that these processes were carried out atthe pressure of 1 atmosphere (close to 100 kPa). If needed, it ispossible to reduce pressure down to 1 kPa with inexpensive vacuum tools.Thus the above processes can be carried out in the gas pressure range of1 kPa to 110 kPa.

[0016] Instead of RF, microwaves can be used for creating plasma atrelatively higher gas pressures (for example 1 kPa or higher), althoughmicrowaves have been used for creating plasma at pressures lower than 1kPa.

EXAMPLE 1

[0017] According to the above process guidelines, the oxidation ofsilicon was carried out in the following manner. Helium gas and oxygengases were introduced in to a chamber in which silicon substrate to beoxidized was placed. The percent of oxygen in the gas mixture was 2%.The pressure was one atmosphere (about 100 kPa). The plasma wassustained by RF power at a frequency of 40 MHz. The RF power density was3 W/cm². The temperature was 200 degree C. The electrode spacing was 1.5mm. The silicon substrate, 0.5 mm thick, was placed on the lowerelectrode. FIG. 2 shows a high frequency C-V curve of a metal oxidesemiconductor (MOS) capacitor fabricated after growing the SiO2 films onp-type silicon using the above conditions. The C-V curve is close to theideal case and has no hysterisis. The defect state density at theSi—SiO2 interface (interface state density or D_(it),) near the siliconmid-gap energy was determined using low-frequency C-V characterization.The D_(it), value was found to be close to 5×10¹⁰ cm⁻²−eV⁻¹. Such a lowvalue of D_(it) indicates an excellent, device-quality interface.

[0018] In order to see how this D_(it) value compares with SiO₂ filmsformed in a similar way using routine low-pressure plasma, oxidationexperiments were carried out at 133 Pa (1 torr) pressure. The otherexperimental conditions were as follows. RF power density=500milli-watts/cm², electrode spacing=20 mm, temperature=200 degree C., andO₂/He=2%. The Dit value obtained for the Si—SiO₂ interface for thislow-pressure plasma oxidation process was 6×10¹¹ cm⁻²eV⁻¹. Thus thedefect density at the Si—SiO₂ interface for the SiO₂ films fabricatedusing the plasma process at pressure substantially close to 1 atmosphereis much lower than that for the SiO₂ fabricated at lower plasma pressureof 133 Pa.

[0019] For comparing the interface state density of the Si—SiO₂interface fabricated by the disclosed method to that of a thermallygrown SiO₂ film, similar silicon wafers oxidized at 1000 □{haeck over(Z)}. The density interface state in the thermal oxidation case wasfound close to 2×10¹¹ cm⁻²eV⁻¹. Thus the defect density at the Si—SiO₂interface for the SiO₂ films fabricated at 200 degree C. using theplasma process at pressure substantially close to 1 atmosphere is alsolower than that for the SiO₂ fabricated at 1000 degree C. using thermaloxidation, indicating high quality of interface obtained using thedisclosed process.

EXAMPLE 2

[0020] In the disclosed process of example 1, the silicon substrate toreact with plasma was placed between the electrodes between which theplasma was created. It is possible to create plasma by applying RF powerin a separate chamber and then transfer the plasma to another chamber inwhich a silicon substrate to react with the plasma is placed. Suchprocess is called remote plasma process and is expected to reduce plasmadamage to the substrate and further improve the quality of the SiO₂films. MOS capacitor were fabricated using the SiO₂ films produced byremote plasma oxidation. For the SiO₂ formation, the pressure was 1atmosphere (about 100 kPa), the temperature was 200 □{haeck over (Z)},the O₂/He gas flow ratio was 1.5%, and the power density was 70 W/cm².FIG. 3 shows the C-V characteristics of the MOS capacitor. In spite ofusing very high power density, no hysterysis is seen in FIG. 3, showinglow damage to SiO₂ films by remote plasma process.

[0021] In the above two examples, oxygen is used as oxidizing agent,however other oxidizing agents such as N₂O or H₂O or a mixture ofvarious oxidizing agents can also be used along with the noble gas tocarry out the oxidation process.

[0022] These SiO₂ films can be subjected to annealing such as thermal,or laser annealing to further improve their properties, preferably in anambient that contains hydrogen such as forming gas ambient.

[0023] It was also observed that addition of small amount offluorine-containing compound (such as HF, NF₃, CF₄) to the process gasmixture enhanced the oxidation rate compared to that observed withoutthe addition of fluorine containing compound. Addition ofchlorine-containing compound is also exhibited similar enhancement ofthe oxidation rate. The addition of fluorine or chlorine compound alsoimproved the film quality.

[0024] The disclosed process can be further refined by adding nitrogenor nitrogen containing compound along with oxidizing agent(s) and noblegas(es) to improve the reliability of device incorporating the silicondioxide films.

EXAMPLE 3

[0025] The SiO₂ film produced by the disclosed process was incorporatedin the fabrication process of TFTs. Two kinds of TFTs werefabricated. 1. Reference TFTs, which were fabricated by a routine TFTprocess and 2. Plasma oxidation TFTs, which were fabricated by thefollowing steps, of which step 3 includes one of the methods related tothe present invention.

[0026] 1. On a glass substrate, amorphous silicon layer with a thicknessof 50 nm was deposited by LPCVD method.

[0027] 2. The amorphous silicon films was laser annealed by a XeClpulsed laser to change it in to polycrystalline silicon with anapproximate grain size of 0.3 micrometers, and the polycrystallinesilicon layer was subsequently patterned by photolithography to makeislands.

[0028] 3. An SiO₂ layer (SiO₂ layer 1) with a thickness of 4 nm wasfabricated over the polycrystalline silicon applying RF power tooxygen-helium gas mixture at pressure substantially close to 1atmosphere (about 100 kPa). An SiO₂ layer (SiO₂ layer 2) was furtherdeposited by ECR CVD method so that the total thickness of layerconsisting of SiO₂ layer 1 and SiO₂ layer 2 was 120 nm.

[0029] 4. Gate metal was deposited and patterned.

[0030] 5. Source and drain regions were created by ion doping.

[0031] 6. An isolation Si0 ₂ layer was deposited and patterned to formsource-drain contact holes.

[0032] 7. Source-drain contact metal was deposited and patterned.

[0033] The reference TFTs for comparison purpose were fabricated by anidentical process mentioned above except for fabricating SiO₂ layer instep 3. The reference TFTs did not contain the said SiO₂ layer 1,instead, the whole SiO2 layer was deposited by ECR CVD having athickness of 120 nm. Thus the reference TFT process is similar to aroutine TFT process, whereas the plasma oxidation TFT process includesan additional SiO₂ layer (SiO2 layer 1) in order to improve theinterface between polycrystalline silicon and the Si0 ₂ layer.

[0034] When the performance of the two kinds of TFTs was evaluated andcompared. the plasma oxidation TFTs performed better than the referenceTFTs in all respect. An n-channel mobility value of 170 cm²V⁻¹sec⁻¹ forthe plasma-oxidation TFTs was obtained compared to a value of 120cm²V⁻¹sec⁻¹ for the reference TFTs. A sub-threshold-slope value of 0.4V/decade for the plasma-oxidation TFTs was obtained compared a value of0.64 V/decade for the reference TFTs. An order of magnitude loweroff-current value for plasma oxide TFTs was obtained compared to thereference TFTs. All these results indicate a significantly betterperformance of the plasma oxidation TFTs.

1. A method to fabricate SiO₂ film comprising a step of applying RFpower in the presence of noble gas(es) and oxidizing gas(es) at totalpressure in the range of 90 kPa to 110 kPa to create a plasma containingreactive oxidizing species, the reactive oxidizing species react withsilicon part of the substrate to convert at least a part of silicon into SiO₂.
 2. The method of claim 1, wherein the said RF power has afrequency in the range of 1 MHz to 100 MHz.
 3. The method of claim 1,wherein said RF power is applied between the electrodes having a spacingbetween them of 5 mm or less.
 4. The method of claim 1, wherein any oneof helium, argon, neon, krypton, xenon or any one of a mixture of atleast two chosen from the group consisting of helium, argon, neonkrypton and xenon is used as noble gas.
 5. The method of claim 1,wherein any one of oxygen, Ozone, H₂O, N₂O or any one of a mixture of atleast two chosen from the group consisting of Oxygen, Ozone, H₂O, N₂O isused as oxidizing gas.
 6. The method of claim 1, wherein the ratio ofpartial pressure of said oxidizing gas(es) to the total gas pressureranges from 0.05 to 15 percent.
 7. The method of claim 1 wherein saidsilicon to be oxidized is placed between the electrodes to which RFpower is applied to create plasma.
 8. The method of claim 9 wherein thesaid RF power density in the range of 0.5 to 10 W/cm2
 9. The method ofclaim 1 wherein said silicon to be oxidized is not placed between theelectrodes between which the plasma is created but the plasma createdbetween the electrodes by application of RF power is subsequentlytransported to the silicon.
 10. The method of claim 9 wherein the saidRF power density is in the range of 1 to 100 W/cm2
 11. The method ofclaim 1, wherein the temperature of the said silicon is in the range of20 degree C. to 700 degree C.
 12. The method of claim 1, wherein thetemperature of the said silicon is in the range of 20 degree C. to 430degree C.
 13. The method to fabricate a SiO₂ film according to claim 1wherein an annealing treatment comprising at least one of thermalannealing, rapid thermal annealing, and laser annealing of the SiO₂ filmis further carried out.
 14. The method of claim 13, wherein the saidannealing treatment is carried out in an ambient containing hydrogen.15. The method of claims 1 wherein the process steps are carried out inthe presence of fluorine-containing compound, the partial pressure ofthe fluorine-containing gas being less than 1 percent of the total gaspressure.
 16. The method of claims 1 wherein the process steps arecarried out in the presence of chlorine-containing compound, the partialpressure of the chlorine-containing gas being less than 5 percent of thetotal gas pressure.
 17. The method of claim 1, wherein the said gasmixture also contains nitrogen or nitrogen containing compound inaddition to noble-gas(es) and oxidizing gas(es).
 18. A metal insulatorsemiconductor field effect transistor (MISFET) device or a thin filmtransistor (TFT) device incorporating SiO₂ film fabricated according tothe method of claim
 1. 19. The method of claim 18, wherein the said SiO₂film is used as gate insulator material or a part of the gate insulatormaterial of a metal insulator semiconductor field effect transistordevice or a thin film transistor device.
 20. A method to fabricatesilicon nitride film comprising a step of applying RF power in thepresence of noble gas(es) and mitriding gas(es) at total pressure in therange of 90 kPa to 110 kPa to create a plasma containing reactivenitriding species, the reactive nitriding species subsequently reactwith silicon part of the substrate to convert at least a part of siliconin to silicon nitride.
 21. The method of claim 20, wherein any one ofhelium, argon, neon, krypton, xenon or any one of a mixture of at leasttwo chosen from the group consisting of helium, argon, neon, krypton andxenon is used as noble gas.
 22. The method of claim 20, wherein any oneof nitrogen, NH₃, N₂O or any one of a mixture of at least two chosenfrom the group consisting of nitrogen, NH₃ and, N₂O is used as nitridinggas.
 23. The method of claim 20 wherein said silicon is placed betweenthe electrodes to which RF power is applied to create plasma.
 24. Themethod of claim 20 wherein said silicon is not placed between theelectrodes between which the plasma is created but the plasma createdbetween the electrodes by application of RF power is subsequentlytransported to the silicon.
 25. The method to fabricate a siliconnitride film according to claim 20 wherein an annealing treatmentcomprising at least one of thermal annealing, rapid thermal annealing,and laser annealing of the silicon nitride film is further carried out.26. The method of claim 25, wherein the said annealing treatment iscarried out in an ambient containing hydrogen.
 27. A metal insulatorsemiconductor field effect transistor (MISFET) device or a thin filmtransistor (TFT)device having a gate insulator material or a part of thegate insulator material made from silicon nitride fabricated accordingto claim
 20. 28. A method to fabricate SiO₂ film comprising a step ofapplying RF power in the presence of noble gas(es) and oxidizing gas(es)at total pressure in the range of 1 kPa to 110 kPa to create a plasmacontaining reactive oxidizing species, the reactive oxidizing speciessubsequently react with silicon part of the substrate to convert atleast a part of silicon in to SiO₂.
 29. A method to fabricate siliconnitride film comprising a step of applying RF power in the presence ofnoble gas(es) and nitriding gas(cs) at total pressure in the range of 1kPa to 110 kPa to create a plasma containing reactive nitriding species,the reactive nitriding species subsequently react with silicon part ofthe substrate to convert at least a part of silicon in to siliconnitride.
 30. A method to fabricate an insulator film comprising a stepof applying microwave power in the presence of noble gas(es) and reagentgas(es) at total pressure in the range of 1 kPa to 110 kPa to create aplasma containing reactive regent species, the reactive speciessubsequently react with silicon part of the substrate to convert atleast a part of silicon in an insulator film.